The present invention relates to a voltage supply circuit of a flash memory device and, more particularly, to a voltage supply circuit for performing a pumping operation simultaneously with the application of a power source and a flash memory device including the same, and a method of supplying an operating voltage.
In semiconductor memory devices, in particular, flash memory devices that enable electrical erasure and programming, Fowler-Nordheim (F-N) tunneling and hot electron injection methods are employed to perform an erase operation for erasing data stored in a memory cell and a program operation for storing data in the memory cell.
In order to program data into a memory cell, the hot electron injection method is used. In a hot electron injection method, electrons of a channel region adjacent to the drain region of the memory cell are injected into a floating gate of the memory cell. A high voltage for programming is applied to the control gate of the memory cell. A high voltage for the operation of a flash memory device generally ranges from 15 V to 20 V. In general, a flash memory device operating at a low power supply voltage includes a voltage supply circuit for generating a high voltage within a chip that includes the flash memory device. The voltage supply circuit is generally configured to pump an input low voltage to a high voltage by employing a voltage pumping circuit.
FIG. 1A is a flowchart illustrating the operation of a conventional voltage supply circuit for supplying a program voltage.
Referring to FIG. 1A, the voltage supply circuit for supplying a voltage for programming a flash memory device cannot supply a voltage sufficient to drive the flash memory device even though an external voltage is supplied to the flash memory device. Thus, any operation should not be performed up to a certain Power-Up Point (PUP). To this end, the external voltage should not be used as an internal voltage of the flash memory device up to the PUP.
If the PUP is reached in step 101, an external voltage is input to the flash memory device and the flash memory device performs initialization for an operation. Since a voltage supply circuit does not start a pumping operation, an output voltage Va has the same level as an external voltage VDD in step S103.
If a command for a program operation is input to the flash memory device and an active command is thus input to the voltage supply circuit in step S105, the voltage supply circuit starts a pumping operation to raise the output voltage Va in step S107. If the output voltage Va is raised by such pumping and the output voltage reaches a desired target voltage in step S109, the pumping operation is finished.
The voltage supply circuit employs the operating current of the flash memory device to obtain a desired output voltage Va. Thus, the pumping operation is performed using most of the operating current of the flash memory device. As the external voltage VDD is applied to the flash memory device, a point where power-up begins is as follows.
FIG. 1B is a view illustrating an operational relationship between an external voltage and the conventional voltage supply circuit.
Referring to FIG. 1B, if the external voltage reaches a certain level (for example, a PUP), the flash memory device is initialized. At this time, in a region A, the voltage supply circuit does not perform any pumping operation. If an active command is input, the voltage supply circuit performs a pumping operation from the external voltage (VDD) level to a desired target voltage using an internal operating current, thereby providing the output voltage Va.
In other words, the voltage supply circuit does not perform an operation for voltage pumping until an idle state or the PUP is reached, but starts the voltage pumping operation after the active command is input. Consequently, most of the operating current of the flash memory device is consumed.